There are a number of applications in which it is of interest to detect or image an object. Detecting an object determines the absence or presence of the object, while imaging results in a representation of the object. The object may be imaged or detected in daylight or in darkness, depending on the application.
Wavelength-dependent imaging is one technique for imaging or detecting an object, and typically involves capturing one or more particular wavelengths that reflect off, or transmit through, an object. In some applications, only solar or ambient illumination is needed to detect or image an object, while in other applications additional illumination is required. But light is transmitted through the atmosphere at many different wavelengths, including visible and non-visible wavelengths. It can therefore be difficult to detect the wavelengths of interest because the wavelengths may not be visible.
FIG. 1 illustrates the spectra of solar emission, a light-emitting diode, and a laser. As can be seen, the spectrum 100 of a laser is very narrow, while the spectrum 102 of a light-emitting diode (LED) is broader in comparison to the spectrum of the laser. And solar emission has a very broad spectrum 104 in comparison to both the LED and laser. The simultaneous presence of broad-spectrum solar radiation can make detecting light emitted from an eyesafe LED or laser and reflected off an object quite challenging during the day. Solar radiation can dominate the detection system and render the relatively weak scatter from the eyesafe light source small by comparison.
Additionally, some filter materials exhibit a distinct absorption spectral peak with a tail extending towards a particular wavelength. FIG. 2 depicts a filter spectrum 200 having an absorption peak 202 and a tail 204 towards the shorter wavelength side. When the wavelengths of interest (e.g., λ1 and λ2) are spaced closely together, it may be difficult to discriminate or detect one or more particular wavelengths. For example, in FIG. 2, the filter material effectively absorbs light at wavelength λ2. But it also partially absorbs light transmitting at wavelength λ1. This can make it difficult to detect the amount of light transmitting at wavelength λ1.